This invention relates to a vehicle brake hydraulic pressure generator of the type in which the output of a booster is adjusted by a control valve, and the thus adjusted output is used to produce a master cylinder pressure, thereby producing a brake hydraulic pressure corresponding to the brake pedal operating amount, and particularly a brake hydraulic pressure generator which can prevent fluctuations in the operating amount (stroke) of the brake pedal or deterioration of the brake pedal operating feeling even when the amount of brake fluid consumed in the brake circuit and/or hydraulic pressure changes. The invention further relates to a vehicle brake hydraulic pressure generator of the above-describe type further having the function of optimizing (quickly increasing) the output hydraulic pressure in the initial stage of brake operation, and/or the function of producing an increased brake hydraulic pressure only with the operating force applied by a driver when the brake booster function has been lost.
The booster in such a brake hydraulic pressure generator amplifies the brake operating force to produce an amplified master cylinder pressure. Such boosters include negative pressure types and hydraulic types. Some of brake hydraulic pressure generators including such a booster are structured such that the brake pedal operating amount directly corresponds to the amount of fluid discharged from the master cylinder. This type of generators have a drawback in that any increase in the amount of brake fluid consumed in the brake circuit at the onset of e.g. anti-lock brake control, regenerative cooperative brake control or vehicle stability control (VSC) has a direct influence on the operating amount of the brake operating means, typically a brake pedal, thus giving uncomfortableness to a driver.
In order to solve this problem, JP patent (unexamined) publication 2002-173016 proposes various brake hydraulic pressure generators which can suppress fluctuations in the brake pedal operating amount even when the amount of brake fluid consumed in the brake circuit fluctuates due to hydraulic pressure control actions not initiated by the driver.
One of the embodiments in this publication (shown in FIG. 4 of the publication) is briefly described here with reference to FIG. 14. More detailed description is found in this publication.
The brake hydraulic pressure generator of FIG. 14 includes a power plate 15a, a valve piston 5b (as a second valve element) received in the power plate 15a so as to be axially movable relative to the power plate 15a, and an input shaft 4 having an integral first valve element received in the valve piston 5b. The brake hydraulic pressure generator further includes a control valve comprising an atmospheric valve seat 5b3 formed on the first valve element, a negative pressure valve seat 5b4 formed on the valve piston 5b, and a valve body 5 fixed to the inner surface of the valve piston 5b for adjusting the output of the power plate 15a by controlling the pressure in a variable pressure chamber (power chamber) 15b. 
When the input shaft is pushed in, a pressure difference is produced between the variable pressure chamber 15b and a constant pressure chamber (negative pressure chamber) 15c. Under this pressure difference, the power plate 15a is pushed toward the master cylinder under an amplified pressure. The master cylinder thus produces brake hydraulic pressure corresponding to the brake operating amount, which is supplied to the brake circuit (wheel cylinders 9). The master cylinder pressure acts on the input shaft 4 as a reaction force.
The strokes of the input shaft 4 and the valve piston 5b are always substantially equal to each other. The input shaft 4 stops when the thrust force applied to the valve piston 5b under the pressure in the variable pressure chamber 15b balances with the force of a spring 7. That is, the stroke of the input shaft 4 is determined by the force of the spring 7. The reaction force that opposes the brake pedal operating force is the master cylinder pressure that is applied to the tip of the input shaft 4. The master cylinder pressure is determined by the pressure in the variable pressure chamber 15b. Thus, the relation between the stroke of the brake pedal and the reaction force applied to the brake pedal are not influenced by and can be set independently of the amount of brake fluid consumed in the brake circuit.
In this arrangement, because the first valve element, which is integral with the input shaft 4, and the second valve element (valve piston 5b), which is axially movable relative to the first valve element, are adapted to move substantially together with each other when the brake pedal is depressed, in order to prevent fluctuations in the brake pedal operating amount due to fluctuations in the amount of brake fluid consumed in the brake circuit, it is necessary to provide a complicated stroke-power converter for controlling the position of the valve piston 5b. This pushes up the cost and size of the entire device.
An object of this invention is to provide a hydraulic pressure generator which can suppress fluctuations in the brake pedal operating amount due to fluctuations in the amount of brake fluid consumed in the brake circuit or fluctuations in the hydraulic pressure with a simple structure, thereby achieving desirable stroke characteristics of the brake operating means.
Also, conventional brake hydraulic pressure generators have no brake pressure jump function, i.e. cannot quickly increase the brake pressure in the initial stage of brake operation, so that it is impossible to optimally control the output hydraulic pressure in the initial stage of brake operation.
Another object of this invention is therefore to provide a brake hydraulic pressure generator which has a brake pressure jump function.
When the booster function has been lost, i.e. if no negative pressure is introduced into the constant pressure chamber 15c, the operating force applied to the brake pedal by the driver had to be transmitted to the master cylinder piston 16a while compressing the spring 7 and the return spring (not shown) for the power plate 15a. That is, part of the force applied to the brake pedal is used to compress the springs, and cannot be fully used to generate brake hydraulic pressure.
A further object of the invention is therefore to provide a brake hydraulic pressure generator which is capable of generating a greater brake hydraulic pressure when the booster function has been lost.